The present invention relates to composite electromagnetic absorbing materials. Electromagnetic absorbing material are used in a variety of applications. They are used in Electromagnetic Compatibility/Electromagnetic Interference (EMC/EMI) test cells to eliminate reflection and interference in the testing. Absorbers are also used in Electromagnetic anechoic chambers for testing high frequency radar, antennas and in Low Observable (LO) structures. The increase in consumer electronics that broadcast, such as cellular telephones and portable computers, have created a new need: the suppression of stray electromagnetic signals in airplanes and near airports to prevent interference with airport radar, communications and automated landing systems. Intelligent Vehicle Highway Systems (IHVS) may also require suppression of electromagnetic signals to prevent multi-path and other types of interference.
Previously, electromagnetic absorbers used only either the electric or the magnetic properties of a material to attenuate the electromagnetic fields. Electric absorption is normally achieved by introducing lossy material, often carbon, to a low dielectric constant material. Examples of this approach include carbon loaded foam and carbon loaded honeycomb. An alternate method is to use specific foam and carbon loaded honeycomb. An alternate method is to use specific patterns of the lossy material to achieve a Debye relaxation of the dielectric constant. See U.S. Patent application Ser. No. 07/890,757 titled xe2x80x9cMethod for Making a Material with Artificial Dielectric Constantxe2x80x9d now U.S. Pat. No. 5,385,623 the disclosure of which is incorporated by reference. The relaxation of the dielectric constant produces a loss in the material that can be accurately controlled in both magnitude and frequency.
Magnetic loss is generally achieved by using a material that exhibits a natural magnetic loss mechanism. Ferrites are a class of material that exhibit this type of loss and are often used in absorbing materials. However, in the frequency range where the ferrites loss is useful, the real part of their relative permittivity and real part of their relative permeability are very different from each other. The result is that the material""s impedance is not close to the impedance of free space and a significant part of the incident energy reflects off the surface. Only when the interference between the surface reflection and reflection from the surface underneath the ferrite cancel each other does the material exhibit its full loss. Therefore, absorbers which use ferrites are effective only over a limited band of frequencies.
The performance of electromagnetic absorbing materials can be improved through grading the electric and magnetic properties within the material and/or by shaping the material. However, even with these techniques, the current state of the art of electromagnetic absorbers results in materials that are either very thick, or work only over a narrow band of frequency. For example, carbon-loaded, foam pyramids used in EMC/EMI test cells are approximately 10 feet long and require ferrite tiles on their base to achieve 10 dB of absorption from 10 MHz to 1 GHz. The size and weight of the pyramids places special requirements on room size and the load bearing capacity of the walls and ceiling.
Thus none of the previous solutions for electromagnetic absorbing materials is totally satisfactory and a need still exists to enhance the performance of electromagnetic absorbers. It is toward the fulfillment of this need that the present invention is directed.
The present invention provides a means and methods to create composite electromagnetic absorbers that provide increased absorption and are thinner and/or lighter than the prior art. To accomplish this, synthetic dielectric materials are combined with either synthetic magnetic materials or magnetically lossy materials in such a way that the permittivity and permeability of the composite material are substantially matched over the desired range of frequency. The match in the permittivity and permeability allows the majority of the electromagnetic fields to enter the material where the electric and magnetic loss components absorb the electromagnetic energy.